Glass-ceramic articles are prepared through the controlled crystallization in situ of parent or precursor glass articles. Hence, the manufacture of glass-ceramic articles comprises three general steps. First, a batch of ingredients in the proper proportions (customarily including a nucleating agent) is compounded and melted. Second, the melt is simultaneously cooled to a glass essentially free from crystallization and an article of a desired geometry shaped therefrom. Third, the glass article is exposed to a heat treatment at temperatures above the transformation range thereof to cause the in situ growth of crystals. Commonly, this heat treatment is divided into two steps. Thus, the glass article will first be heated to a temperature within or slightly above the transformation range to develop nuclei therein, after which the article will be heated to a considerably higher temperature, frequently in excess of the softening point of the glass, to cause the growth of crystals on the nuclei.
Inasmuch as the crystallization process contemplates the essentially simultaneous growth of crystals on countless nuclei dispersed throughout the precursor glass article, the microstructure of a glass-ceramic article customarily consists of relatively uniformly-sized, fine-grained crystals homogenously distributed, but randomly oriented, within a residual glassy matrix. Conventionally, the crystal phase constitutes the predominant proportion, i.e., greater than 50% by volume, of a glass-ceramic article. Such high crystallinity normally leads to glass-ceramic articles exhibiting chemical and physical properties quite different from those of the precursor glass body and more closely resembling those of the crystal phase. Furthermore, the residual glassy matrix will be small in amount and of a very different composition from that of the parent glass, since the constituents making up the crystal phase will have been removed therefrom during the crystallization process. Finally, the presence of a residual glassy matrix renders the glass-ceramic article free of voids and non-porous.
U.S. Pat. No. 2,920,971, the basic patent in the area of glass-ceramics, provides an extensive discussion of the mechanism of crystal growth and the practical considerations involved in the manufacture of glass-ceramic articles. Reference is made to the patent for a general understanding that the crystal phases developed in glass-ceramic articles and the amount of such crystallinity are dependent upon the parent glass composition and the heat treatment parameters to which the parent glass is exposed.